Page 103 - Adsorption Technology & Design, Elsevier (1998)

100 Processes and cycles

separated from the fluid by sedimentation, filtration, etc. either for disposal or
for reuse. Powdered activated carbon (PAC) is often used in this way to
remove tastes and odours from waters. If sufficient time is allowed for
equilibrium to be reached then the loading of the adsorbate on the adsorbent
will be related to the final concentration of the adsorbate in the solution via the
thermodynamic isotherm which applies at the final temperature in the process.
Powdered or granular adsorbents are usually added to the equipment in
slurry form in such a way as to allow adequate dispersion and mixing. The
adsorbent can be removed as a settled sludge. When large quantities of
adsorbent are required consideration should be given to using a multiple
batch or cross-flow system. For example, one way of reducing the total
amount of adsorbent required is to carry out the batch processing in two
steps, as shown in Figure 5.3. The feed is first contacted with a fresh batch of
adsorbent. After separation of the fluid from the adsorbent the fluid is
contacted with a further fresh batch of adsorbent. Each subsequent batch of
adsorbent removes less and less impurity as the concentration of the
impurity in the fluid decreases. The overall mass balance equations which
describe batch adsorption processes are given in Chapter 6. Equations which
describe the dynamics of batch adsorption are provided in Chapter 4.

Transfer olution
s

\

First batch of Second batch of
~k ads~ / ~k~. ads~ /

Figure 5.3 Basic equipment ]'or the contacting of a liquid with two batches of
adsorbent.